1. Field of the Invention
The present invention relates to a power amplifier apparatus, and more particularly to, a power amplifier apparatus in which an operational amplifier works as a PWM (pulse width modulation) amplifier as well as an analog amplifier according to an input signal level.
2. Description of the Prior Art
FIG. 3 is a block diagram showing a conventional power amplifier apparatus. In the figure, denoted 1 is an input terminal to which an input signal is inputted, 2 is a PWM amplifier for amplifying the input signal inputted to the input terminal 1, and 3 an analog amplifier for amplifying the input signal which is inputted to the input terminal 1.
A reference numeral 4 shows an input signal level detector for detecting the level of the input signal inputted in the input terminal 1. When the level of the input signal exceeds a selected level, driving signals are outputted during the input signal level is larger than the selected level. On the other hand, when the input signal level becomes lower than the selected level, non-driving signals are outputted during the input signal level is smaller than the selected level.
Numeral 5 shows a changeover switch. A contact 5a is connected to an output terminal of the PWM amplifier 2, and a contact 5b to an output terminal of the analog amplifier 3. Further, a movable contact 5c is switched between the contacts 5a and 5b with the output of the input signal level detector 4.
Numeral 6 shows a low-pass filter (LPF), of which end is connected to the movable contact 5c of the changeover switch. Denoted 7 is a load connected to the other end of the low-pass filter.
Next, the operation of the power amplifier apparatus will be explained.
An input signal is inputted to the input terminal and the input signals are amplified by the PWM amplifier 2 or the analog amplifier 3. Then, the input signal level detector 4 detects the level of the input signal. When the input signal level is lower than the selected level, the movable contact 5c of the changeover switch 5 is changed to the contact 5b to out put a non-driving control signal and when the input signal level is larger than the selected level the movable contact 5c is changed to the contact 5a to output a driving control signal.
As a result, when the input signal level is lower than the selected level, the input signal is amplified by the analog amplifier 3 and is inputted to a load 7 by way of the changeover switch 5 and the low-pass filter 6. When the input signal level is larger than the selected level, the input signal is amplified by the PWM amplifier 2 and is inputted to the load 7 through the changeover switch 5 and the low-pass filter 6.
The cut-off frequency of the low-pass filter 6 is set, for example, between 20 kHz and 50 kHz so as to cut off a carrier through the PWM amplifier 3. As a result, even if the output of the analog amplifier is provided to the low-pass filter 6, is does not have bad effect on the output of the low-pass filter 6. Such power amplifier apparatus as described above is disclosed in U.S. Pat. No. 5,077,539.
In the conventional power amplifier apparatus with the above configuration, an amplifier which is operated with the level of the input signal is changed between a PWM amplifier and an analog amplifier, which requires two line with the PWM amplifier 2 and the analog amplifier 3, resulting in rather large-sized apparatus.
Further, unless the PWM amplifier 2 and the analog amplifier 3 are not provided with the same characteristics, the characteristics of the PWM amplifier 2 and the analog amplifier 3 becomes discontinuous when one of the amplifiers is changed to the other amplifier, causing a trouble such as a noise.